Solid particulates, which include but are not limited to, mine tailings, ores, sand, coal, soils, and aggregates, have conventionally been transported through pipelines or boreholes as a slurry. Typically, a liquid, such as water, is added to the particulates to make the particulates flow. The particulates are suspended in a sufficient quantity of liquid such that the flow of the liquid moves the particulates through the pipeline.
Ideally, the particulates should remain in suspension while being transported through pipelines or boreholes to their final destination. Particulates, such as classified or unclassified mine tailings, may be characterized by features that limit the ability to transport the material as a paste. Such characteristics may include: high density or specific gravity, poor or gapped gradation, coarse particle size, highly sorted particle size gradation, or lack of fine particles. These features tend to prevent particulates from being made into a stable paste consistency without the addition of fine materials, such as sands, to enhance the particle size distribution.
As a result, these particulates must currently be transported as low-load slurries that must maintain turbulent flow conditions throughout the transport system to prevent the particulates from settling out and clogging the transport pipes. Additionally, the transport fluid may bleed out of the slurry and cause reduced fluidity of the mixture and clogging of the transport pipes.
Reduction of fluid content may produce a material that approaches the consistency of a paste, but which would also develop reduced turbulence and shear thickening properties. This makes transport by pumping or by gravity through a transport system less desirable and more problematic than by hydraulic transport.
One type of particulate transport is the transport of mine tailings as backfill in mines, or to the surface to remove the tailings from the mine. The backfill mine tailings can be transported alone, or a cement can be added to provide for a set composition. It is desired in the art to transport mine tailings as a paste consistency, rather than by hydraulic transport. However, the particle size distribution needs to be optimized to provide paste consistency transportation. The particle size distribution of a mine tailing may be made to conform to a more idealized particle size distribution by the addition of fine materials or sand. This however increases the handling and treatment costs of the mine tailings and increases costs to the mining operations. The ideal fill exhibits slurry-like properties of ease of transport in pipes (i.e., low pressure losses), but paste-like properties of staying in suspension at relatively low transport velocities with no run off.
Once the slurry of particulates arrives at the desired destination, the transport fluid drains out. The fluid must then be pumped away, which adds additional handling and disposal costs to the operation. Additionally, if a cement is included with the particulates, some cement will be present in the drain water which may then harden in the drain system, and increase the maintenance costs of the operation. In the cases where mine tailings are transported to the surface, a dam must be erected to manage and recover the water. This adds additional cost, potential liability, and is a potential safety hazard.
Foam suspends the particulates and reduces the amount of liquid required to transport the material. It is known, as in U.S. Pat. No. 5,080,534 to Goodson et al., to generate a foam from a surfactant and then mix the foam with the particulates prior to transport in the pipeline. Also, it has been known, as in U.S. Pat. No. 4,451,183 to Lorenz, to inject foam that was separately prepared, into the particulates at various points throughout the pipeline. However, these prior art methods require that the foam be separately prepared before being added to the mixture. The separate preparation requires additional equipment and time to prepare the foam, which adds additional cost to the project.
Mine backfilling is the process of filling voids in mines left by the underground excavation of ore to provide structural support for the mine or to dispose of mine tailings. Materials used to fill mine voids include waste rock, mine tailings, and sand. Often, sand is mixed in with mine tailings.
Mine tailings are the result of grinding the ore or rock fine enough to liberate minerals through a flotation process. Because of the low content of minerals in the ground rock, typically less than 10% by weight, almost all of the rock mined becomes mine tailings. Mine tailings generally have consistencies similar to those of fine clay or silt. Traditionally, large quantities of water must be added to the tailings to make them pumpable. The tailings at final placement then must be thickened to allow the tailings to set in place. There is a very significant cost to the thickening of mine tailings.
Once the mine tailings are pumped from the mill, they are either sent directly to a tailings pond, sent directly to a backfill plant, or processed with a hydro-cyclone to separate the coarse particles from the finer particles. The coarse fraction is sent to the backfill plant, and the fine fraction is sent to the tailings pond. A tailings pond is generally a large area where the tailings are left to settle and the resulting water is drained off to clarification ponds.
A method of producing mine backfill from mine tailings is to process the tailings slurry with a bank of hydro-cyclones to remove the coarser portion of the material. The coarser portion (underflow) is fed into a silo and allowed to settle with some of the water overflowing the top of the silo. The denser tailings material is removed from the silo and pumped to a mixer, where cement is added to produce hydraulic backfill.
The backfill, which is a slurry of typically 50% solids by weight, is poured down a vertical hole or pipe to be deposited in the mining voids or stopes left underground. Because it is a slurry, the backfill requires high volumes of cement to consolidate the material, and water that does exude from the backfill must be pumped out of the mine. The cement contents range from about 3% by weight to about 10% by weight. Using a relatively low percentage of cement requires a much longer time for the backfill mass to be considered "load-bearing" or competent.
Another method of backfill is paste backfill. Paste backfill uses a lower cement content, usually about half, as compared to the hydraulic backfill method described above. Also, there is no water exuded from the fill mass to pump to the surface. The consolidation time is reduced to a fraction of the time required to consolidate hydraulic backfill, allowing a faster cycle time. Another advantage of using paste backfill is that it is usually made from total (unclassified) tailings. This reduces the burden of maintaining a large surface impoundment area.
Paste backfill, however, has some drawbacks. It is more difficult to transport paste to the mine or stopes, sometimes requiring a positive displacement pump to transport the paste through the piping. The production of paste is usually accomplished by processing the mill tailing slurry by using large vat thickeners, disc filters, or a combination of both. This results in a large multimillion dollar capital investment.
What is needed in the art is a foaming agent that can be incorporated into the solids mixture, to then form the foam within the mixture as a single-stage process, to eliminate the separate stages of preparation, pregeneration, and combination of a foam into the slurry prior to transport of the solids as in a pipe.
It is therefore an object of the invention to provide a method of transporting particulates by incorporating a foaming agent in a mixture of particulates and then producing a foam in the mixture prior to transport through pipelines or boreholes as a slurry.